The contacts of electrical switches operating under load typically erode during normal operation and even further deteriorate when overheating occurs. Erosion and overheating of the contacts can cause failures or deteriorated switch operation and otherwise generally reduce or limit the useful lives of the switches themselves. The degree of erosion or deterioration from overheating is a function of the various conditions that exist during operation, such as the amount of current carried by the contacts, the voltage applied across the contacts, the maximum operating temperature experienced, along with the severity of service under which the contacts operate (e.g. the amount and frequency of switching operations). In addition, erosion or overheating of electrical contacts can signal failure or malfunction of other switch components.
The erosion of electrical contacts most commonly results from the arcing which occurs whenever a switch breaks a circuit. An arc is formed as the electrical contacts move apart from each other and the electric potential between them causes electrons to bridge the intercontact space region. A current is maintained in the arc until the spacing between the contacts, and thus the impedance, increases enough to prevent electrons from bridging the gap for the given voltage potential. The current flowing across the gap generates heat, resulting in temperatures high enough to burn away some of the contact material. Switches may fail when their contacts have eroded so far that they cannot effectively complete a circuit.
Switches are also subject to overheating from a high resistive contact interface. Excessive heating of contacts or other switch components can less dramatically change the physical characteristics of the contacts than erosion, but nonetheless can cause significant contact deterioration and even contact failure in the long run. Among other things, overheating can cause the contacts to become brittle and/or excessively carbonized which can result in a type of failure known as a "flash-over" failure within the switch.
Electrical contacts have a useful life which is related to the extent of erosion or overheating, if it occurs. Once a contact has eroded to the point in which further use risks injury to personnel or machinery, known as the "critical point," a contact's useful life is over. The critical point is a measure of volume and is reached when, as a result of erosion for example, only a predetermined percentage of a contact remains.
Because arcing and erosion cannot be eliminated, switches are often designed to allow replacement of the contacts. It is typically less expensive to replace worn contacts than to replace an entire switch when the contacts have eroded to the critical point or close thereto. As a result however, users of switches must monitor the erosion of the contacts to recognize when the predetermined critical point is approaching or has been reached. Replacing worn contacts at or before the critical point is important because contacts used past that point continue to erode and may cause the switch to fail. A switch failure can have a negative or catastrophic effect on equipment and presents a danger to personnel. On the other hand, replacing contacts before the end of their useful life increases material and labor costs. Monitoring of the temperature to which components have been subjected is also helpful in assessing the efficiency of operation and remaining useful life of components, such as switch contacts, even before a failure such as a "flash-over" occurs.
There are four basic environments within which electrical contacts operate: (1) in air, (2) in inert gas, (3) under oil, and (4) within a vacuum. Each of these environments presents challenges to the contact monitoring process.
Air-environment contacts can be observed visually to monitor the degree of wear, allowing replacement at times appropriate to the life of the contact before the risk of failure is inordinately great. Inert gas-environment and vacuum-environment contacts usually cannot be observed visually, as they are most often contained in an opaque enclosure or vacuum bottle. Oil-environment contacts are used for medium and high voltage equipment, including circuit breakers and transformer and regulator load tap changers used by electric utilities. These contacts operate under oil in an enclosed tank or compartment, preventing easy access to the contacts. Regardless of the type of environment in which contacts and other components operate, they may be operated in some form of enclosure. For air or oil environments, this enclosure may be open to the atmosphere, but for vacuum or inert gas environments, the enclosure must be sealed. Sealed enclosures make monitoring particularly difficult.
A transformer has two sets of wire coils, known as the primary windings and the secondary windings. A voltage applied to the primary windings (known as the primary voltage) will induce a voltage in the secondary windings (known as the secondary voltage). The secondary voltage will be higher or lower than the primary voltage, depending upon the relationship of the number of turns, or coils, of wire in the primary and secondary windings of the transformer. A transformer with a greater number of coils in the secondary windings will produce a secondary voltage higher than the primary voltage. A transformer without taps, or access points, in the secondary windings will produce only one secondary voltage for each primary voltage. Many examples of transformers have numerous taps in the secondary windings so a variety of secondary voltages may be selected from one transformer. A transformer which has taps in the secondary windings will allow several secondary voltages to be accessed, depending upon which tap is selected. One transformer may be used to both decrease and increase voltage, if it is tapped at points lower and higher in number than the number of turns in the primary windings. Means known as a "coil tap selector switch" or a "load tap changer" must be provided, however, to switch between the various secondary winding taps.
A "load tap changer" is a mechanical device that moves an electrical contact to different taps within the transformer or regulator, depending on the voltage output required. In some designs, the electrical contact is moved while current is still flowing within the transformer or regulator, creating numerous instances of arcing across the load tap changer's contacts as they move from one tap position to the next. In other designs, a transfer switch is employed to transfer the current during switching. In this case, the transfer switch uses a large sacrificial contact that is designed to perform the function of making and breaking the current, and arcing occurs on the sacrificial contact.
There is a large expense associated with shutting down and opening these types of equipment to determine the extent of wear or erosion of the contacts. This expense is compounded by the necessity of removing, storing, and processing a large quantity of oil, sometimes up to 1000 gallons. Contacts are often replaced early due to the difficulty of predicting the rate of erosion from one maintenance cycle to the next. The expense of inspecting the contacts is often so great that maintenance departments invariably change the contacts during every inspection, even though the contacts may have months or more of useful life remaining. Properly matching the timing of inspection with the end of the useful life of the contacts would thus advantageously result in a cost savings.
Some of the means used previously to monitor electrical equipment performance which attempted to overcome the effort and expense required by direct physical inspection include the following:
1. Dissolved Gas Analysis (DGA)
Dissolved gas analysis is used in an oil environment. In DGA, a sample of the oil surrounding the contacts is extracted and analyzed to monitor for dissolved gases. The presence of dissolved gases is indicative of various types of problems that may be occurring within the equipment. For example, the presence of acetylene dissolved in the oil surroundings is indicative of core failure in transformers. This process lacks the precision necessary to determine the proper timing of contact replacement, as the presence of gas is neither directly related to the amount of erosion of the contacts nor an indication of the degree of contact heating.
2. Infrared Monitoring
Infrared monitoring may be used in an air, inert gas, vacuum, or oil environment. In infrared monitoring, an infrared camera is used to monitor the temperature of high voltage equipment. Temperature and resistance are directly related. As resistance to current flow through electrical equipment increases, the temperature of the equipment and its surroundings also increases. The infrared camera measures in a general sense the temperature increases and alerts the user accordingly. However, this system is insufficient because it does not measure erosion and is not sufficiently accurate to monitor the temperature of contacts or other components separately from other neighboring components within the enclosure.
There is accordingly a need to provide an electrical contact wear and heating indicator which will automatically monitor contacts and provide an indication to users that the critical point or one or more temperatures have been reached.